1. Field of the Invention
The present invention relates to a light modulation information display device (hereinafter referred to as an “LM information display device”) which displays information through variable control of the transmission, absorption, interception, reflection state or reflection direction of light, and an illumination control device for controlling an illumination device which is provided on a back face or a front face of a display section of an LM information display device. In particular, the present invention relates to an LM information display device and an illumination control device which can provide improved power consumption and improved display quality for moving pictures, and higher reliability. Moreover, the present invention relates particularly to: an LM information display device which can be suitably used as a liquid crystal display device for displaying moving pictures or the like; and an illumination control device which is used as a backlight control device for controlling a backlight provided on a back face of a display section of such an LM information display device, or as a frontlight control device for controlling a frontlight provided on a front face of such an LM information display device, and which can achieve optimum ON/OFF control for a fluorescence discharge tube, e.g., a cold-cathode fluorescence discharge tube.
2. Description of the Related Art
An LM information display device which incorporates an illumination device and an illumination control device for controlling the illumination device can have various structures. Examples of such LM information display devices include underlying-type backlight LM information display devices and side-type backlight LM information display devices. Such classification is based on the positioning of the illumination device.
In the field of transmission liquid crystal display devices, which are exemplary of LM information display device currently in use, it is commonplace to employ an underlying-type backlight LM information display device in order to improve the display uniformity. This is especially the case with large-size transmission liquid crystal display devices (i.e., of a size designated as “20” or higher) for displaying moving pictures. Hereinafter, as Conventional Example 1, an example of a conventional underlying-type backlight LM information display device and a conventional side-type backlight LM information display device will be described.
FIG. 20 schematically shows a conventional underlying-type backlight LM information display device 2000. The underlying-type backlight LM information display device 2000 includes an LM information display section 2001, illumination devices (fluorescence discharge tube) 2003 and 2014, and a light guide layer 2002 for guiding illumination light emitted from the fluorescence discharge tubes 2003 and 2014 into the LM information display section 2001.
In the underlying-type backlight LM information display device 2000, the fluorescence discharge tubes 2003 and 2014 are provided directly under the light guide layer 2002, so that the underlying-type backlight LM information display device 2000 itself may have a relatively large depth. However, the thickness of the underlying-type backlight LM information display device 2000 does not increase with an increase in the number of fluorescence discharge tubes 2003 and 2014. Moreover, the underlying-type backlight LM information display device 2000 provides a greater flexibility as to the number and arrangement of fluorescence discharge tubes 2003 and 2014 to be employed than a side-type backlight LM information display device.
FIG. 21 schematically shows a conventional side-type backlight LM information display device 2100. The side-type backlight LM information display device 2100 includes an LM information display section 2111, a light guide layer 2112 for guiding light into the LM information display section 2111, lamp reflectors 2116a for deflecting the light toward the light guide layer 2112, and at least one fluorescence discharge tube 2116 which is partially surrounded by the lamp reflector 2116a. Although the lamp reflectors 2116a and the fluorescence discharge tubes 2116 are illustrated as being provided on both sides of the light guide layer 2112 in the side-type backlight LM information display device 2100 of FIG. 21, a lamp reflector 2116a and a fluorescence discharge tube 2116 may be provided on only one side of the light guide layer 2112.
In the case where the above side-type backlight LM information display device is employed for a large-size display devices for displaying moving pictures, it is commonplace to increase the number of fluorescence discharge tubes 2116 to be provided on either side or both sides in order to obtain improved luminance and to alleviate luminance unevenness. In this case, however, the size of the display device 2100 itself increases in proportion with the number of fluorescence discharge tubes 2116 employed.
In general, a backlight control device is controlled so as to be always ON in the following manner. A DC rated voltage is input to an inverter circuit, and a high step-up ratio is obtained by means of a piezoelectric transformer at the beginning of the discharging in order to begin discharging of the fluorescence discharge tubes. Once discharging is begun and the impedance of the fluorescence discharge tube has lowered, a stable voltage is obtained by means of a winding transformer so as to maintain the fluorescence discharge tube to be ON.
In recent years, it has been discovered through line-of-sight tracing tests that display blurs, e.g., blurred outlines, occur with a hold-type emission display method (as used in liquid crystal display devices, etc.), as opposed to an impulse-type emission display method (as used in CRTs (cathode ray tubes), etc.), thereby detracting from the display quality when displaying moving pictures.
FIG. 22A shows results of line-of-sight tracing with respect to a hold-type emission display method. In FIG. 22A, the axis of ordinates represents time, where one resolution unit is equal to 1/60 sec, which corresponds to 1 frame period; and the axis of abscissas represent the positions of pixels.
In this case, since the illumination device is always ON during 1 frame period, a viewer's eyes will try to follow a movement in the display with a locus as indicated by the broken lines in FIG. 22A. As a result, the viewer will see an image in accordance with an integral of the luminance values and relative positions along the broken lines. Therefore, the viewer cannot capture the proper gray-scale images (portions indicated in black), but instead sees an image which is a combination of the proper gray-scale images and any gray-scale values (portions indicated in dots) adjoining the outline. Such portions contribute to so-called blurred outlines.
One conventional approach for improving such display blurs involves the use of ON periods and OFF periods within 1 frame period, in an attempt to realize a CRT-like impulse-type emission display method.
FIG. 22B shows results of line-of-sight tracing with respect to a case where ON periods and OFF periods are present within 1 frame period of an illumination device. In this case, during frame transitions, the gray-scale components associated with the adjoining pixels do not contribute to the trace line (indicated by the broken lines) with which the line-of-sight of a viewer follows positions on the outline. As a result, the viewer is prevented from seeing an image having blurred outlines.
In order to implement an impulse-type emission display method in a liquid crystal display device (which is an exemplary LM information display device), it might be possible to operate a display panel of the liquid crystal display device so as to obtain bright or dark images while controlling the fluorescence discharge tubes so as to be always ON. However, obtaining bright or dark images based on the operation of a liquid crystal display device is accompanied by the following problems.
Firstly, an increase in the power consumption in the liquid crystal display device results, thereby detracting from its comparative advantages over other types of display devices (CRTs, PDPs (plasma display panels), etc.). Secondly, since there is an increased number of fluorescence discharge tubes with a high density, the temperature of the fluorescence discharge tubes may increase as a result of controlling the fluorescence discharge tubes so as to be always ON, resulting in a decrease in display contrast. Thirdly, there is a problem associated with the response speed, which is dependent on the particular liquid crystal material used: outstanding display blurs (e.g., blurred outlines) and residual images will occur when moving pictures are displayed at a fast rate.
Another possible method for implementing an impulse-type emission display method in a liquid crystal display device involves flickering a fluorescence discharge tube(s) composing a backlight. The following conventional backlight control device structures for controlling such a backlight have been proposed. For example, Japanese Laid-Open Publication No. 3-198026 (filed by Hitachi, Ltd.) adopts a technique of “splitting a backlight into a plurality of regions, such that the split regions can be controlled so as to flicker and/or have controlled luminance in a distinguishable manner”. Japanese Laid-Open Publication No. 11-297485 (Sony Corporation) adopts a technique of “inactivating an inverter circuit during a blanking period of an image signal so as to turn off fluorescence discharge tubes used as a backlight”.
Referring to FIG. 20, it will be described how such conventional techniques can be implemented in the operation of the aforementioned conventional LM information display device 2000 (which is an underlying-type backlight LM information display device). The light guide layer 2002 is split into a plurality of regions, and the fluorescence discharge tubes 2003 and 2014 are provided on the back face of the light guide layer 2002 so as to correspond to the respective split regions of the light guide layer 2002. The fluorescence discharge tubes 2003 and 2014 are configured so as to be capable of flickering (or having controlled luminance) simultaneously or individually for the respective split regions. The fluorescence discharge tube 2003 (indicated in white) represents a fluorescence discharge tube which is ON (or has a high luminance), whereas the fluorescence discharge tubes 2014 (indicated in black) represent fluorescence discharge tubes which are OFF (or have a low luminance).
The aforementioned conventional examples can be commonly characterized in that, instead of turning all of the fluorescence discharge tubes ON or OFF, illumination devices (fluorescence discharge tubes) are controllable so as to be individually turned ON or OFF or have their light amounts regulated (bright or dark) based on an image signal for the display device, thereby improving the power consumption of the device.
In the aforementioned Conventional Example 1, cold-cathode fluorescence discharge tubes are used as fluorescence discharge tubes. Since the electrode structure in cold-cathode fluorescence discharge tubes does not require a filament transformer mechanism, unlike the electrode structure in hot-cathode discharge tubes, cold-cathode fluorescence discharge tubes are advantageous in terms of power consumption, device life/reliability, and down-sizing. Hence, cold-cathode fluorescence discharge tubes are employed as illumination devices in many liquid crystal display devices.
The electrode structure in a conventional cold-cathode fluorescence discharge tube is essentially a two-terminal discharge tube structure. The ON/OFF control of the cold-cathode fluorescence discharge tube is performed via an inverter circuit in a such a manner that a DC voltage is stepped up at the beginning of the discharging by means of a step-up means so as to instantaneously generate a discharge starting voltage for the fluorescence discharge tube. Thereafter, after the impedance of the fluorescence discharge tube has lowered, a stable voltage is generated by means of a winding transformer, whereby the ON state is maintained.
The discharge starting voltage has an excessive voltage component as compared to the ensuing discharging voltage. It is known that, since the amount of electrons which are sputtered increases at the beginning of the discharging, vigorous sputtering occurs in the neighborhood of the electrodes, leading to the blackening of the fluorescent material and the deterioration of the electrodes.
A method for establishing a stabilized discharging has been proposed, which involves the use of cold-cathode fluorescence discharge tubes having a multi-electrode structure (Conventional Example 2). For example, according to Japanese Laid-Open Publication No. 4-342951 (Sony Corporation), an auxiliary electrode is provided in the neighborhood of two main discharging electrodes of a cold-cathode fluorescence discharge tube, so that a potential difference can be obtained between the main discharging electrodes and the auxiliary electrode at the beginning of the discharging. Thus, a stable discharge state can be obtained in a short period of time.
As described above, in transmission liquid crystal display devices, which are exemplary conventional LM information display devices, cold-cathode fluorescence discharge tubes are generally employed from the perspective of power consumption, device life/reliability, and down-sizing, and an always-ON method is used as the ON/OFF control method thereof.
While the aforementioned technique of repeatedly turning ON or OFF the fluorescence discharge tubes as illustrated in Conventional Example 1 does contribute to an improvement in power consumption, it is disadvantageous in terms of the device life of the fluorescence discharge tubes. This is because, at each moment when a fluorescence discharge tube transitions from an OFF state to an ON state, impulse noises such as an undershoot may be added in an inverter circuit which serves as an ON/OFF control circuit for the fluorescence discharge tubes, so that the instantaneous potential difference may exceed the rated input voltage value for the inverter circuit. Consequently, excessive components may be applied to the fluorescence discharge tubes as a discharge starting current and a discharge starting voltage. Thus, the amount of electrons which are sputtered increases at the electrodes of the fluorescence discharge tubes, resulting in a vigorous sputtering and leading to the blackening of the fluorescent material and the deterioration of the electrodes. This shortens the device life of the fluorescence discharge tubes.
Furthermore, in accordance with a light regulation method which repeats transitions between bright/dark states by controlling the luminance of the fluorescence discharge tubes, there can be an improvement in the power consumption of no more than about 20% to 30% (by actual measurement values). This technique also has a problem, among others, in that a substantial increase in temperature occurs in the case where fluorescence discharge tubes are provided close together; when such a high temperature is transmitted to the liquid crystal panel, the display contrast is decreased, undermining the display quality and reliability.
In conventional fluorescence discharge tubes having a multi-electrode structure described in Conventional Example 2, in which an increased number of electrodes are employed in the cold-cathode fluorescence discharge tubes so as to stabilize the initial discharging, strong electron bonds are present between the main discharging electrodes at the beginning of the discharging. As a result, the amount of electrons which are sputtered increases between the auxiliary electrode and the main discharging electrodes, leading to electrode deterioration.
Furthermore, the conventional method in which the interference of image information associated with the adjoining display frames is prevented by flickering the fluorescence discharge tubes during 1 frame period of displaying information in order to improve the display blurs of LM information display devices has a problem in that the number of times that the fluorescence discharge tubes are switched, i.e., the number of times that the discharge starting voltage is applied, increases. As a result, the device life of the fluorescence discharge tubes may drastically deteriorate.